FIG. 34 shows an example of a conventional thin, planer acoustic transducer. This planer acoustic transducer includes a yoke 50; a plurality of bar-shaped magnets 52 which are arranged in parallel on the yoke; a vibrating diaphragm 54 which is arranged in parallel with the pole faces of the bar-shaped magnets 52; and a plurality of coils 56 which are arranged on the vibrating diaphragm 54 at positions that face the bar-shaped magnets 52 such that current flows in a direction perpendicular to a magnetic field generated from the bar-shaped magnets 52. When alternating current is caused to flow through each of the coils 56, in accordance with Fleming's left hand rule, force is generated between the coil 56 and the magnetic field. As a result, the vibrating diaphragm 54 is vibrated in a direction perpendicular to the surface of the diaphragm, and electrical signals are converted into sound signals.
However, the aforementioned planer acoustic transducer involves problems, including generation of noise resulting from twisting of the vibrating diaphragm due to force along the surface of the diaphragm, which force occurs by the effect of the magnetic field perpendicular to the coils on the diaphragm surface, as well as low degree of freedom in the design of planer acoustic transducer shape or coil impedance, since, for example, the coils facing the bar-shaped magnets have an elongated rectangular shape, and most portions of the coils are located in regions facing the pole faces of the bar-shaped magnets.
In an attempt to improve the planer acoustic transducer of FIG. 34 involving the aforementioned problems, a planer acoustic transducer having a configuration shown in FIG. 35 has been proposed. In the planer acoustic transducer having this configuration, a plurality of magnets 62 are arranged on a yoke 60, with the magnets being in parallel with a vibrating diaphragm 64, such that the pole faces of adjacent magnets differ from each other. Furthermore, a plurality of spiral coils 66 are arranged on one surface or both surfaces of the vibrating diaphragm 64 at positions facing the pole faces of the magnets 62, such that the innermost circumference of each of the spiral coils is located in the vicinity of a portion of the vibrating diaphragm, the portion corresponding to the outer periphery of each of the pole faces. In FIG. 35, reference numeral 68 denotes a damper.
With the configuration shown in FIG. 35, force which the coils receive from a magnetic field perpendicular to the vibrating diaphragm is reduced, and generation of noise is suppressed. In addition, the portions of the coils that are perpendicular to a magnetic field in parallel with the vibrating diaphragm are increased in area, whereby sound conversion efficiency is enhanced, and the degree of freedom in the design of planer acoustic transducer shape or coil impedance is increased as compared with the case of the planer acoustic transducer of FIG. 34.
In the aforementioned conventional planer acoustic transducers, generally, coils are formed on a vibrating diaphragm by means of the below-described method. Specifically, through holes are formed, by means of drilling, through-hole plating, etc. in a substrate sheet prepared by forming a metallic layer on both surfaces of a resin-made film (e.g., a polyimide film or polyester film) through a technique such as sputtering, plating, or application of metallic foil, or in a composite sheet prepared by bonding metallic foil (e.g., copper foil or aluminum foil) onto a substrate such as a prepreg formed by impregnating glass cloth, aramid non-woven fabric, or the like with an epoxy resin, a thermosetting polyester resin, or the like; and subsequently, unnecessary portions of the metallic foil are removed by means of a process similar to that employed for producing a printed wiring board, such as etching, to thereby form coils.
Alternatively, the below-described method is employed for forming coils on a vibrating diaphragm. Specifically, a coil pattern and through-holes (conducting portions) for electrically conducting circuits on both surfaces of a substrate are formed, by means of metal plating, directly on the substrate, such as a sheet prepared by thermally curing a resin-made film (e.g., polyimide film or polyester film) or a prepreg formed by impregnating glass cloth, aramid non-woven fabric, or the like with an epoxy resin, a thermosetting polyester resin, or the like.
The vibrating diaphragm produced through the above-described method generally has a configuration as shown in FIG. 36. In FIG. 36, reference numeral 70 denotes a substrate film, 72 a coiled circuit, and 74 a through-hole connection portion.
However, the aforementioned conventional coil formation methods involve problems. Specifically, in the case of the method in which through-holes are formed in a film substrate having a metallic layer on both surfaces thereof, and then coils are formed through etching (among printed wiring board production methods, this method is called a “subtractive method”), under some etching conditions, a portion of the coils may be excessively etched, and the width of conductors constituting the coils may be reduced, leading to an increase in impedance and, in the worst case, occurrence of circuit breakage. Meanwhile, this method tends to cause problems, including a decrease in impedance, which results from occurrence of short circuit between adjacent conductor or an increase in the width of the conductors due to insufficient etching.
In the case of the method in which coils are formed directly on a substrate by means of metal plating (among printed wiring board production methods, this method is called an “additive method”), for example, difficulty is encountered in maintaining uniform thickness of conductors in all the coils during plating of the coils; i.e., the degree of freedom in the design of planer acoustic transducer impedance becomes low.
Each of the aforementioned conventional methods also involves problems in that a complicated process is required for production of a vibrating diaphragm, variation in the impedance of the thus-produced vibrating diaphragm is large, and production cost is high.
In the case where coils are formed by means of the subtractive method or the additive method, difficulty is encountered in arbitrarily designing the area of the cross section of coils under mass production conditions, since some limitations are imposed on the etching conditions or the plating conditions. Furthermore, in the case where coils are formed by means of the subtractive method or the additive method, since coils cannot overlap with one another on a single substrate surface, the degree of freedom in the design of impedance becomes low, and the cross-sectional area of spiral coils fails to be increased to more than 0.02 mm2.
FIGS. 37(A) through 37(C) show an example of a conventional planer acoustic transducer. In the figures, reference numeral 110 denotes a flat yoke formed of an iron plate (ferromagnetic metallic plate), 112 a plurality of permanent magnets which are mounted on one surface of the yoke 110 such that the magnetic axes are perpendicular to the yoke surface, and 114 a vibrating diaphragm. The permanent magnets 112 are mounted on the surface of the yoke 110 at predetermined intervals such that the pole faces of adjacent magnets are of opposite polarity. The vibrating diaphragm 114 includes an insulating base film 116, and spiral voice coils 118 which are formed on both surfaces (or one surface) of the base film 116 such that the respective voice coils correspond to the respective permanent magnets 112. All the voice coils 118 are connected together such that current flows in the same direction at the adjacent sides of adjacent voice coils. Reference numeral 126 denotes a coating film for covering the voice coils 118.
The yoke 110 has holes 124 for regulating change in air pressure, which is caused by vibration of the vibrating diaphragm 114. The periphery of the vibrating diaphragm 114 is connected, via an elastic supporting member 128, to a yoke stepped portion 110b provided on a yoke peripheral wall 110a, and the vibrating diaphragm 114 is movably supported at a desired distance from the pole faces of the permanent magnets 112. A buffer sheet 130 is provided between the vibrating diaphragm 114 and the permanent magnets 112, so that the vibrating diaphragm 114 does not come into contact with the pole faces of the permanent magnets 112. The buffer sheet 130 may be a sheet formed of a highly resilient material, so as not to impede vibration of the vibrating diaphragm 114. Reference letter G denotes a gap between the vibrating diaphragm 114 and the buffer sheet 130, and reference numeral 122 denotes an input terminal, 132 an insulating plate, 134 an external terminal, and 136 a flexible conductor.
The aforementioned planer acoustic transducer can be configured into a thin form.
However, when the aforementioned planer acoustic transducer is used for a long period of time, metal fatigue tends to occur in the voice coils formed on the insulating base film, leading to wire breakage of the coils, since the voice coils themselves vibrate during use of the planer acoustic transducer. Metal fatigue occurs as a result of repeated application of stress to particular portions of a metallic material.
In addition, in the case of the aforementioned planer acoustic transducer, since the insulating base film, which serves as a substrate of the vibrating diaphragm, has a very small thickness; i.e., about 4 to about 100 μm, a sharp trough of sound pressure occurs within a midrange of 300 to 800 Hz, leading to deterioration of sound quality.
Furthermore, in the case of the aforementioned planer acoustic transducer, since the voice coils are provided on the vibrating diaphragm, Joule heat generated from the voice coils is readily transmitted to the vibrating diaphragm, possibly leading to degeneration of the vibrating diaphragm. Also, the vibrating diaphragm may deflect under its own weight and come into contact with the surface of the magnets, leading to deterioration of characteristics of the planer acoustic transducer.